MOS (metal-oxide-semiconductor) varactors are semiconductor devices that have a capacitance varying as a function of an applied voltage. Varactors are often used as tuning elements in circuits such as voltage controlled oscillators (VCOs), parameter amplifiers, phase shifters, phase locked loops (PLLs), and other tunable circuits. For example, by varying a voltage applied to a varactor, the frequency of operation of an associated VCO can be adjusted. Tunability, linearity, and quality factor are among the important characteristics of an MOS varactor.
As the semiconductor industry strives for higher device density, higher performance, and lower power, problems involving MOS varactor fabrication and design have been encountered, particularly with FinFET varactors in advanced process nodes such as 16 nanometer (nm) or smaller. For example, it has been found that FinFET varactors sometimes do not provide good tunability and linearity for ultra-low power circuits. For example, a linear tuning region of a FinFET varactor may be centered on a gate voltage (Vg) greater than 0 (zero) volts such as 0.3V, and may become almost un-tunable when Vg swings near 0V. That means a VCO circuit needs to provide a Vg biasing near 0.3V, while an ideal Vg biasing is near 0V for achieving ultra-low power consumption in the circuit. Accordingly, improvements in FinFET varactor design and fabrication are desired.